1. Field of the Invention
The present invention relates to a package structure for housing an optical element and gas-hermetically sealing optical fibers connected to the optical element, and a composite structure of an optical element and optical fibers housed and gas-hermetically sealed in the above-mentioned package structure.
The package structure and the optical element-optical fiber composite structure of the present invention are useful for optical devices comprising an optical waveguide electro-optical element.
2. Description of the Related Art
In a conventional optical element-optical fiber composite structure in which an optical element is housed and gas-hermetically sealed in a package, a pair of optical fibers are connected to the optical element and extended to the outside of the package through sleeves attached to the package, and the sleeves are gas-hermetically sealed. Usually, each optical fiber comprises a naked core fiber, and coating layers comprising a primary coat layer formed on the naked core fiber and a secondary coat layer formed on the primary coat layer. When the optical fiber is connected to the optical element, at an end portion of the optical fiber, the coating layer is removed so as to expose the naked core fiber to the outside, and a portion of the naked core fiber adjacent to a portion of the optical fiber coated with the coating layer or the primary coat layer is surface-metallized with a metal, for example, nickel and gold. A terminal face of the naked fiber portion is connected to a terminal face of the optical element through an adhesive, and the surface-metallized fiber portion is bonded to the sleeve with a moisture-nonpermeable bonding material, for example, solder. Namely, a gap between the peripheral surface of the surface-metallized fiber portion of the optical fiber located in the sleeve and an inside peripheral surface of the sleeve is sealed by the solder.
Usually, the soldered surface-metallized fiber portion of the optical fiber exhibits a relatively low tensile strength of 0.5 to 1.5 kgf. An ideal tensile strength of the core fiber is about 6 kgf. Reasons for the reduction in the tensile strength of the soldered core fiber portion are assumed that microcracks are unavoidably formed on the naked core fiber portion while the coating layer is removed, a surface metallization is applied to the naked core fiber portion surface and soldering is applied to the surface-metallized core fiber portion.
Therefore, it is practically difficult to enhance the tensile strength of the soldered portion of the surface-metallized core fiber portion. However, this tensile strength, namely a seal-fixing strength of the optical fiber to the package, is unsatisfactory.
In a prior art, an attempt was made in which a portion of the optical fiber having the coating layer is inserted into the sleeve and the coating layer is bonded to the inside peripheral surface of the sleeve, to enhance the seal-fixing strength of the optical fiber to the package.
Nevertheless, this attempt is disadvantageous in that generally, the coating layer and the core fiber are significantly different in expansion coefficient from each other, namely the core fiber has an expansion coefficient of one tenth (1/10) or less that of the coating layer, the coating layer retains a stress generated during the formation thereof, and therefore, when a heating and cooling are cyclically applied to the optical fiber, the coating layer is expanded and shrunk to an extent larger than that of the core fiber. If the interfacial bonding strength between the coating layer and the core fiber is low, it appears that the core fibers are pushed out from the coating layer when the coating layer shrinks and are pulled into the coating layer when the coating layer expands. The pushing and pulling distance of the core fiber due to the above-mentioned phenomenon is variable depending on the type and the material of the coating layer. When the secondary coat layer is made of a polyamide resin, the pushing and pulling length of the core fiber sometimes reaches more than several hundred .mu.m within temperature range of -20.degree. and 70.degree. C. Also it is practically impossible to eliminate this phenomenon.
In the above-mentioned type of sealing manner of the optical fiber to the sleeve, the optical fiber is fixed at two portions thereof spaced from each other, namely at the soldered portion of the surface-metallized fiber portion and the adhesive-bonded portion of the coating layer-coated portion. Where the core fiber pushed out from the coating layer, a non-fixed portion of the core fiber between the two fixing points is bent in the sleeve, and sometimes is broken by a compression buckling thereof due to a buckling stress (localized tensile stress) generated therein.
In practical use, the surface of the core fiber located in the sleeve is sometimes gradually corroded, and thus the buckling stress generated in the core fiber causes the corroded core fiber to be broken.
Accordingly, there is a strong demand for preventing the generation of the buckling stress in the core fiber.
To prevent the breakage of the core fiber due to the buckling stress, it has been attempted to make the distance between the fixing points of the optical fiber large enough to absorb and relax the deformation of the core fiber. However, this attempt was not successful because the large distance between the fixing points causes the resultant package structure for the optical element and the optical fibers to have too large a size.